Chronic exercising dogs will be used to study the accuracy of ventilatory control during exercise. Arterial blood gas tensions will be fixed by controlling the inspired gas mixture. The interaction of the chemical stimuli; hydrogen ion, carbon dioxide and oxygen with each other and with exercise will be quantified for each animal studied. The ability of increases in ventilation with exercise to precisely match increases in oxygen consumption in the absence of chemical feedback, termed the accuracy of the exercise response, will be determined. A highly accurate response is indicative of a response operating in the closed loop with some quantity sensed and fed back, whereas a less accurate response suggests that the control is open loop. Individual differences in the accuracy and in the role and interactions of chemical stimuli will be of particular interest. Once the response of an animal is well characterized, specific studies will attempt to alter the observed relationships. Alterations in the capacity of the blood to carry oxygen and CO2 will be examined. Controlled reductions in the blood flow to exercising muscle using an occluder in series with a flowmeter will be performed. Long term training, forcing the animals to exercise under altered conditions of gas exchange, caused by altering the program of the inhaled gas controller, will be used to search for a slow adaptive component of the exercise response. One of the specific hypotheses that these studies will attempt to test is that different subjects respond to exercise in different manners, with varying strength and possibly with varying contributions from each of several stimuli responsible for the ventilatory response to exercise.